So, this is in fact the Jailer.
A lot of us were hoping this wasn't the Jailer and the silhouette we saw in the trailer was a WIP. Sure, it was a silhouette, but the two look pretty different even taking that into account. To me it seems pretty cringey when the official trailer of the expansion contains an incomplete depiction of a character, especially the primary antagonist. I mean, what if others looked different in game as compared to their appearance in their respective expansion's trailer? Illidan in BC, Arthas in Wrath, Deathwing in Cata, all of those characters looked the same in the trailer which is what convinces us of the trailer's importance. The trailer to a work of art is often created after most of the art is finished, that way they are depicting an accurate representation of the art they are advertising. That makes sense, doesn't it? Feel free to share your stance on this issue, but it is ultimately not the purpose of this thread.
The following is heavy speculation. You've been warned!
I don't know about anybody else, but to me the Jailer looks like the offspring of an Orc and Nathrezim. And the Jailer has got to be god-tier powerful, essentially being the Satan of the WoW universe. Is his appearance the form he takes out of choice or is it a natural form he never chose? Given we know they're in league with one another, is it possible that Sire Denathrius was created or transformed by the Jailer to give his Dreadlord-like appearance? It's an interesting concept, considering that Revendreth and the Maw both serve a similar purpose: Containing the souls of evil-doers. Only one is intended to rehabilitate the soul whereas the other is eternal damnation. Furthermore, are the Nathrezim like, living beings that have been corrupted by Denathrius? Or maybe they've always been like that and they were created for the very purpose of the Venthyr having a representation in the material universe. And if the Nathrezim are in fact related to the Denathrius and the Venthyr, then they are also indirectly linked to the Jailer himself. So could the Dreadlords have been under the (perhaps indirect) orders of the Jailer the entire time? The entire time meaning the 40,000k+ years they were connected to the Burning Legion, or perhaps even before the Burning Legion, when the demons were a disorganized threat constantly being contained by the Titans.
The Nathrezim stole the Helm of Domination and Frostmourne, or something along those lines, correct? They were created in Torghast, i.e. by the Jailer's blacksmith, certainly under his orders. That throws a wrench in the works. Why would the Nathrezim steal the creations of their master? Well, I think they "stole" these artifacts under Sire Denathrius' orders, not knowing that the Jailer intended for this to happen so that they would be brought into the universe and used in the way they did. See how Sylvanas breaking the Helm of Domination opened the shattered glass-looking tear between the universe and the Shadowlands? It's as if the Helm of Domination was the only thing keeping the two realms apart. Perhaps the Nathrezim brought the Helm of Domination to the universe, and to Azeroth, so such a link would be created between the two realms. If that's the case, Sargeras was played big time by the Dreadlords and by extension, the Jailer. The LK and the Scourge therefore could have been because of the Jailer's machinations (which raises questions about Maldraxxus as well), then because of that Sylvanas happened, who ended up as the Jailer's champion.
My dudes, this goes deep. Let me know what you think.